The invention relates to a method of fabricating substrates, in particular for optics, electronics or optoelectronics.
The starting materials used in fabricating the substrates can be obtained industrially, for example in the form of ingots of raw material.
In the case of monocrystalline silicon, for example, those ingots are obtained using the CZOCHALSKI pulling (CZ pulling) method from a bath of fused silicon or using a zone melting (ZM pulling) method from a polycrystalline ingot.
Monocrystalline silicon carbide is fabricated, for example, by means of a sublimation method that is well known to the skilled person.
Such growth methods produce ingots that are generally cylindrical in shape with two substantially conical ends. Depending on the nature of the material constituting the ingot, it may be from about ten centimeters long (for silicon carbide, for example) up to about 2 meters long (for silicon).
The ingots are then cut into slices perpendicular to the longitudinal axis of the cylinder to form later wafers of starting material that are then used in a variety of applications. The wafers are only a few hundred micrometers (μm) thick (as an example, 200 millimeter (mm) diameter silicon wafers of standard fabrication are 725 μm thick).
The steps for fabricating such wafers typically consist of trimming the two pointed ends of the ingot, and then grinding and turning its irregular lateral surface to obtain a cylinder of perfectly circular cross section. Next, the ingot is cut into slices using, for example, a circular saw or a wire saw.
The rondelles or slices obtained then undergo finishing, which includes grinding them to obtain a wafer with a uniform thickness, and then polishing at least one of its two opposite faces to obtain a perfectly flat surface. Finally, each wafer is immersed in a series of chemical baths to eliminate the dust and particles that may still subsist on the two faces and which could be a source of subsequent pollution.
The above-mentioned steps are extremely expensive both as regards the cost of the equipment used to carry out the machining or treatments, and as regards the time taken to execute and the loss of starting material on cutting. For example, when a 300 μm thick wafer is to be produced, about 400 μm of material is lost in forming that wafer. Thus, for an ingot length of 1 cm, i.e., 10000 μm, it is only possible to produce fourteen wafers (fourteen times 700 μm).
Further, with certain extremely hard, fragile and brittle materials such as silicon carbide, the above-mentioned preparation steps can prove to be extremely long and tedious as the product takes a long time to polish and chemical attack is difficult.
Moreover, sometimes the massive wafers obtained merely constitute an intermediate product. In certain methods of removing and transferring thin layers, for example a method such as that described in U.S. Pat. No. 5,374,564, only the front face of the wafer acts as a base for removing material and thus only the front face must be rigorously flat. In contrast, it is pointless to subject the rear face of the wafer and the cylindrical lateral face to polishing and finishing that is expensive both as regards time and starting material.
The goal of the invention is to overcome the disadvantages described above and, in particular, to substantially reduce the loss of starting material from the materials used in producing the substrates, in order to reduce fabrication costs.
The invention also aims to simplify and limit the machining steps carried out on the starting ingot to as great an extent as possible.